Comparing Interior Routing Protocols
Two types Distance Vector
◦ Examples: RIP v1 and RIPv2 (Routing Information Protocol) IGRP (Interior Gateway Routing Protocol)
Link State◦ Examples
OSPF (Open Shortest Path First) IS-IS (Intermediate System - Intermediate System) NLSP (Netware Link Services Protocol)
Path Vector◦ Example
BGP (Border Gateway Protocol)
Review: Interior Gateway Protocols
Link State vs. Distance VectorLink State (LS) advantages:
More stable (aka fewer routing loops)
Faster convergence than distance vector
Easier to discover network topology, troubleshoot network.
Routing table shows the◦ route source
Directly connected networksStatic routesDynamic routing protocols
Parent and Child Routes◦ A Level 1Parent route does not contain any next-hop IP address or exit interface information
Level 2 child routes contain route source & the network address of the route
Diagram illustrates 2 child networks belonging to the parent route 172.16.0.0 / 24
Review: Routing Table Structure
level 1 route
level 2 route
Longest Match: Level 1 Network Routes–Best match is also known as the longest match –The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.
Routing Table Lookup Process
Classful routing protocols do not send subnet mask information with their routing updates.
A router running a classful routing protocol will react in one of two ways when receiving a route:◦ If the router has a directly connected interface belonging to the
same major network, it will apply the same subnet mask as that interface.
◦ If the router does not have any interfaces belonging to the same major network, it will apply the classful subnet mask to the route.
◦ Example: 10.3.1.0 and 10.5.5.0 belong to the same major network (10.0.0.0)
What happens when Router B sends routing update to Router A?◦ What subnet mask will be used by Router A?
Classful and Classless Routing
When using classful routing protocols, the subnet mask must remain consistent throughout your entire network
Scenario: Routing Behaviour- Classful/Classless Routing
What happens to a packet with destination 172.16.4.0/24 for • Classful routing and • Classless routing?
If no match is found in child routes of previous slide then router continues to search the routing table for a match that may have fewer bits in the match
Routing Behavior: Classless Routing
why the router drops the Packet destined to 172.16.4.0/24None of the child routes left most bits match the first 24 bits.
Classful Routing Behavior
Route Summarization
Figure 1
The use of CIDR and VLSM not only reduces address waste, but it also promotes route aggregation, or route summarization.
route summarization reduces the burden on upstream routers.
Example: Figure 1 variable-sized networks and
subnetworks is summarized at various points using a prefix address until the entire network is advertised as a single aggregate route of 192.168.48.0/20
Route Flapping
Figure 1
Route flapping occurs when a router interface alternates rapidly between the up and down states. This can be caused by a number of factors, including a faulty interface or poorly terminated media.
Summarization can effectively insulate upstream routers from route-flapping problems.
Example: Figure 1 If the RTC interface connected
to the 200.199.56.0 network goes down, RTC removes that route from its table
What if routers were not configured to summarise?
Steps to calculate a route summary
List networks in binary format
Count number of left most matching bits to determine summary route’s mask
Copy the matching bits and add zero bits to determine the summarized network address
Classless Inter-Domain Routing (CIDR)
Default routes ◦ Packets that are not defined specifically in a
routing table will go to the specified interface for the default route
◦ Example: Customer routers use default routes to connect to an ISP router.
◦ Command used to configure a default route is
◦ #ip route 0.0.0.0 0.0.0.0 s0/0/1
Default Route
Default Route and Static route
When network topology changes, network traffic must reroute quickly. The phrase "convergence time" describes the time it takes a router to start using a new route after a topology changes.
Routers must do three things after a topology changes:
Detect the change
Select a new route
Propagate the changed route information
Comparing Routing Protocols: Convergence
EASE OF IMPLEMENTATION
SPEED OF IMPLEMENTATION
Comparing Routing Protocols: ADMINISTRATIVE CRITERIA
Three key issues determine the amount of bandwidth a routing protocol consumes:
1. When routing information is sent---1. Periodic updates are sent at regular intervals.
Flash updates are sent only when a change occurs.
2. Complete updates contain all routing information. Partial updates contain only changed information.
3. Flooded updates are sent to all routers. Bounded updates are sent only to routers that are affected by a change.
Note: These three issues also affect CPU sage.
Comparing Routing Protocols BANDWIDTH REQUIREMENTS
CPU usage is protocol dependent. Some protocols use CPU cycles to compare new
routes to existing routes. Other protocols use CPU cycles to regenerate
routing tables after a topology change. In most cases, the latter technique will use
more CPU cycles than the former. ◦ Example: For link-state protocols, keeping areas small
and using summarization reduces CPU requirements by reducing the effect of a topology change and by decreasing the number of routes that must be recomputed after a topology change.
Comparing Routing Protocols:CPU REQUIREMENTS
Routing protocols use memory to store routing tables and topology information.
Route summarization cuts memory consumption for all routing protocols.
Keeping areas small reduces the memory consumption for hierarchical routing protocols
Comparing Routing Protocols: MEMORY
The ability to extend your internetwork is determined, in part, by the scaling characteristics of the routing protocols used and the quality of the network design.
Network scalability is limited by two factors: ◦ operational issues and
Operational scaling concerns encourage the use of large areas or protocols that do not require hierarchical structures.
◦ technical issues When hierarchical protocols are required, technical
scaling concerns promote the use of small areas
Comparing Routing Protocols: SCALABILITY
Some routing protocols provide techniques that can be used as part of a security strategy.
Some routing protocols allow filter on the routes being advertised so that certain routes are not advertised in some parts of the network.
Some routing protocols can authenticate routers that run the same protocol. Authentication mechanisms are protocol specific
Authentication can increase network stability by preventing unauthorized routers or hosts from participating in the routing protocol, whether those devices are attempting to participate accidentally or deliberately.
Comparing Routing Protocols: SECURITY
RIP: Routing Information Protocol Uses hop count as metric (max: 16 is infinity) Tables (vectors) “advertised” to neighbors every
30 s. Each advertisement: upto 25 entries
No advertisement for 180 sec: neighbor/link declared deadroutes via neighbor invalidatednew advertisements sent to neighbors
(Triggered updates)neighbors in turn send out new advertisements
(if tables changed)link failure info quickly propagates to entire netpoison reverse used to prevent ping-pong
loops (infinite distance = 16 hops)
RIPv1 Problems (Continued) Split horizon/poison reverse does not
guarantee to solve count-to-infinity problem◦ 16 = infinity => RIP for small networks only!◦ Slow convergence
Broadcasts consume non-router resources RIPv1 does not support subnet masks
(VLSMs) ◦ No authentication
RIPv2 Why ? Installed base of RIP routers Provides:
◦ VLSM support◦ Authentication◦ Multicasting◦ “Wire-sharing” by multiple routing domains,◦ Tags to support EGP/BGP routes.
Uses reserved fields in RIPv1 header. First route entry replaced by authentication
info.
E-IGRP (Interior Gateway Routing Protocol)
CISCO proprietary; successor of RIP (late 80s) Several metrics (delay, bandwidth, reliability, load
etc) Uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg.
(DUAL) based on diffused computation Freeze entry to particular destination Diffuse a request for updates Other nodes may freeze/propagate the
diffusing computation (tree formation)Unfreeze when updates received.Tradeoff: temporary un-reachability for some
destinations
Link State Protocols Key: Create a network “map” at each node. 1. Node collects the state of its connected links
and forms a “Link State Packet” (LSP) 2. Flood LSP => reaches every other node in the
network and everyone now has a network map. 3. Given map, run Dijkstra’s shortest path
algorithm (SPF) => get paths to all destinations 4. Routing table = next-hops of these paths. 5. Hierarchical routing: organization of areas, and
filtered control plane information flooded.
Link State Issues Reliable Flooding: sequence #s, age LSA types, Neighbor discovery and
maintainence (hello)◦ Efficiency in Broadcast LANs, NBMA, Pt-Mpt
subnets: designated router (DR) concept Areas and Hierarchy
◦ Area types: Normal, Stub, NSSA: filtering◦ External Routes (from other ASs), interaction with
inter-domain routing. Advanced topics: incremental SPF
algorithms
OSPF
OSPF Network Topology OSPF Addressing and Route Summarization
OSPF Route Selection
OSPF Convergence
OSPF Network Scalability
OSPF Security
Next Week..
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